For the purposes of fabricating electronic, optoelectronic semiconductor and microelectromechanical (MEMS) devices in silicon, it is often necessary to perform liquid anisotropic etching of the silicon through either silicon dioxide (SiO2) or silicon nitride (Si3N4) masks, whereby the silicon crystal planes such as (100), (110) and (111) planes are etched at different rates. The structures resulting in the silicon following such liquid anisotropic etching processes can consist of pyramidal mesa frustum shapes, inverted pyramidal cavities as well as other geometries. Wet etchants that have been used for etching silicon preferentially along crystallographic planes include aqueous solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), cesium hydroxide (CsOH). Aqueous solutions of tetramethylammonium hydroxide (TMAH, (CH3)4NOH) and tetraethylammonium hydroxide ((C2H5)4NOH) have also been used for liquid anisotropic etching of silicon. Other substances that are known to etch silicon anisotropically, include ethylene diamine pyrocatechol (EDP) and hydrazine (N2H4). Each of the aforementioned liquid anisotropic etchants have drawbacks for application in the anisotropic etching of silicon. The alkali metal hydroxides cannot be used for fabricating electronic and optoelectronic silicon devices due to the nature of the alkali metal ion that acts to degrade the silicon dioxide dielectric material properties in MOS and CMOS type structures. The alkali metal hydroxides can therefore only be utilized for liquid anisotropic etching of silicon in MEMS applications. The anisotropic liquid etchant TMAH does not contain alkali metal cations and can be prepared with sufficient purity to support liquid anisotropic etching of silicon for electronic and optoelectronic device applications. The drawbacks of using TMAH include its toxicity, making it difficult to handle as well as difficult to dispose of, falling under the category of a hazardous waste. The liquid anisotropic etchant EDP, is a very effective etchant but extremely corrosive and even more toxic and carcinogenic than TMAH. It has to be treated and disposed of as a hazardous waste which makes it difficult and costly to use. Hydrazine (N2H4) also functions as a liquid anisotropic etchant for silicon but is extremely corrosive, toxic and carcinogenic, thereby complicating its use and making it costly due to the problem of disposal as a hazardous waste. Moreover, hydrazine is an extremely flammable liquid, having very high energy content, making its principal application as a component in fuels for rocket and jet engines. The existing liquid anisotropic etchants have major drawbacks for application to silicon, with TMAH being the least problematic of the ones described for fabrication of silicon electronic and optoelectronic devices.
The present invention describes a method for implementing liquid anisotropic etching of silicon for the full range of applications including silicon electronic and optoelectronic devices as well as silicon MEMS device fabrication using ammonia (NH3) gas dissolved in high purity deionized water, to form the aqueous base ammonium hydroxide (NH4OH) which acts as the anisotropic etchant. An overpressure of NH3 gas is maintained within the hermetically enclosed etching apparatus, to prevent the dissolved ammonia gas from evaporating from the solution at the elevated temperatures required to effect a high anisotropic etching rate of the silicon in the aqueous NH4OH solution. The principal advantage of using aqueous NH4OH over other methods to anisotropically etch silicon includes the capability of preparation in an extremely pure form at the point of use by dissolving ultra high purity (99.9999%) semiconductor grade ammonia (NH3) gas into distilled/deionized water that contains the silicon wafer substrate which must be etched. The ammonium hydroxide anisotropic etching solution, similar to TMAH, EDP and hydrazine, does not contain alkali metal cations and therefore can be used for silicon microelectronic device fabrication as well as for MEMS fabrication. In addition, neither the ammonia gas nor the aqueous ammonium hydroxide (NH4OH) solution are as corrosive, toxic or carcinogenic as TMAH, EDP or hydrazine and therefore, require only normal precautions for handling. The spent aqueous ammonium hydroxide solution can be easily neutralized with a weak acid and does not constitute a hazardous waste, making disposal environmentally friendly and therefore, far less costly to use.
To effectively use aqueous ammonium hydroxide for anisotropic etching of silicon, the solution must be maintained at a temperature between 70-90° C. At these temperatures however, the dissolved ammonia will evaporate rapidly from a solution heated in the open atmosphere, thereby diminishing the concentration of ammonium hydroxide in the aqueous solution and inhibiting the etching action of silicon. To prevent evaporation of ammonia from the aqueous NH4OH solution at the optimal etching temperature of 70-90° C. and thereby reducing the NH4OH concentration in solution, an apparatus must contain the etching solution within a hermetically sealed chamber with an overpressure of NH3 gas maintained above the NH4OH solution at a pressure level above the normal atmospheric pressure. An overpressure of NH3 gas above the NH4OH liquid anisotropic etching solution that prevents further evaporation of NH3 from the solution can be created in one example approach by dissolving a predetermined weight of ammonia gas into a set volume of deionized water contained in a polytetrafluoroethylene (PTFE) beaker inside the hermetically enclosed pressure chamber at room temperature, to form a fixed and known concentration solution of NH4OH. The temperature of the NH4OH solution is subsequently raised between 70-90° C. to increase the etch rate of the silicon. Some ammonia will evaporate from the solution at the elevated temperature required for etching, however, the sealed pressure chamber prevents its escape beyond the volume of the apparatus. The pressure in the hermetic chamber increases as more ammonia evaporates from the NH4OH solution, eventually reaching an equilibrium steady state between the rate of NH3 evaporation from the aqueous NH4OH solution into the enclosed chamber and NH3 dissolving back into the solution. The equilibrium will occur at a NH3 gas pressure above normal atmospheric pressure, and it is for this reason that special apparatus is required for the liquid anisotropic etching method, that is capable of withstanding the pressure at equilibrium of NH3 above the aqueous NH4OH solution, as well as the corrosive effects of NH3.
Although it has been possible to perform liquid anisotropic etching of single crystal silicon using aqueous solutions of alkali metal hydroxides, TMAH, EDP and hydrazine, where for example the (111) crystallographic plane of silicon is etched at a slower rate compared to the (100) and (110) silicon planes, application of these etchants is very much limited due to the contaminating effects of the alkali metal cations to silicon dioxide in MOS and CMOS electronic device structures. For the case of TMAH, and especially EDP and hydrazine, the corrosive, carcinogenic and environmentally hazardous nature of the chemicals requires special safety precautions, making them costly to use. To date, no effective method with supporting apparatus, exists or has been described, for performing very high purity liquid anisotropic etching of silicon, suitable for electronic, optoelectronic and MEMS device applications while supporting etch rates comparable with the aforementioned existing anisotropic etchants and using instead, environmentally clean and minimally hazardous liquid anisotropic etchants. In contrast to the existing technology for liquid anisotropic etching of silicon using alkali metal hydroxides, TMAH, EDP and hydrazine, the versatile method and apparatus of the present invention supports the use of a very high purity aqueous NH4OH solution with elevated NH3 pressure (overpressure) above the aqueous NH4OH solution, prepared at the point of use, from two precursors including distilled/deionized water held in a pure fluoropolymer (PTFE) material beaker and very high purity (99.9999%) semiconductor grade NH3 reacting together in a specially designed and constructed corrosion resistant nickel alloy hermetic chamber, to form the very high purity aqueous NH4OH solution for liquid anisotropic etching of the silicon. The apparatus consisting of the specially designed and constructed corrosion resistant nickel alloy hermetic chamber, allows the aqueous NH4OH anisotropic etching solution to be heated to an optimal temperature between 70-90° C. to enable a high etching rate of the single crystal silicon or polycrystalline silicon, by preventing the ammonia from evaporating and escaping from the liquid anisotropic aqueous NH4OH etching solution. The byproducts of liquid anisotropic silicon etching according to the method of the present invention, include NH3 gas and unreacted aqueous NH4OH solution containing consumed silicon hydroxides. These substances are environmentally friendly by virtue of being easy to neutralize and are minimally hazardous in contrast to TMAH, EDP, and hydrazine.
As illustrated in U.S. Pat. No. 6,787,052, the method proposed for deep etching of single crystal silicon wafers for fabrication of microstructures within the silicon relies on a first etching step using dry reactive ion etching (RIE) followed by a liquid anisotropic etching step using the well known in the art liquid anisotropic etchants, alkali metal hydroxides, tetramethylammonium hydroxide (TMAH), ethylene diamine pyrocatechol (EDP), gallic acid or hydrazine. The liquid anisotropic etching step of the described method for deep etching of single crystal silicon wafers, does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH3 gas from aqueous NH4OH solution for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 5,976,767, the method proposed for selectively etching polysilicon using ammonia solution or aqueous NH4OH is described, that is selective to silicon dioxide and photoresist. The exposed polysilicon gate which is usually deposited in thin layers of a few tens of nanometers on various substrates, is etched by the aqueous NH4OH solution having a 1-5% concentration by volume in water, and maintained at a low temperature between 20-30° C. The described etching method, although using aqueous NH4OH solution as the etchant, applies the technique to etching isotropically, only thin layers of polysilicon and is not appropriate for anisotropic etching of single crystal silicon having a thickness of several thousand nanometers. Moreover, the method, does not describe a solution or apparatus that enables increasing the anisotropic etch rate of single crystal silicon or polysilicon using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH3 gas from aqueous NH4OH solution for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 5,431,777, the method for crystallographically selective etching or anisotropic etching of silicon is presented in the presence of p-doped silicon where part of the silicon is dissolved, while a p-doped pattern in the surface remains largely undissolved. The anisotropic etchant composition of the described method that leaves p-doped silicon largely unetched consists of an aqueous solution of alkali metal hydroxide or tetraalkylammonium hydroxide and a high flashpoint alcohol, phenol, polymeric alcohol or polymeric phenol. The described anisotropic etching method for silicon, does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 5,296,093, the method for anisotropically etching a masked polysilicon layer formed over a step on an integrated circuit structure and having oxide portions is presented. The invention describes treating the integrated circuit structure after the polysilicon etch, with an aqueous ammonium-containing base mixed with peroxide solution to selectively remove the polymeric silicon/oxide-containing residues remaining after anisotropic etching of the polysilicon layer. The described anisotropic etching method for polysilicon does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 5,207,866, the method for anisotropically etching single crystal silicon is described by placing it in an etching solution consisting of R4NOH and solvent wherein R is an alkyl group having between 0 and 4 carbon atoms. The solution will preferentially etch <100> or <110> oriented single crystal silicon, additionally, electrochemical etching may be employed to preferentially etch p-type single crystal silicon. The described anisotropic etching method for single crystal silicon does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 5,071,510, the method for electrochemical etching of silicon wafers or plates is described whereby the wafer front-side has a monocrystalline epitaxial layer having a doping type opposite to the remainder of the silicon wafer thereby forming a p/n junction. An organic photoresist film protects the epitaxial layer on the wafer front-side or epitaxy side so that the etchant composed of tetraalkylammonium hydroxide in water solution or in water-free form will etch the wafer back-side and a small voltage bias applied to the junction from the front-side assures an etch-stop at the p/n junction. The described anisotropic etching method for silicon wafers does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 4,765,865, the method for increasing the etch rate of a single crystal silicon wafer in anisotropic etching solution by applying a masking layer to part of one face of the wafer and a metal coating to the other face of the wafer making the wafer more anodic than that of only a masked single crystal silicon wafer. Furthermore, an external potential can be applied to the masked and metalized wafer to increase the etching rate on the masked side as long as the potential is less than that which will cause the potential to exceed the passivation potential of a masked single crystal silicon wafer. The described anisotropic etching method for silicon wafers does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 4,172,005, the method of etching a semiconductor substrate is described which comprises the steps of mounting a mask for etching on the semiconductor substrate and effecting crystallographically selective etching using an anisotropic etchant comprising an aqueous solution of 0.1-20% by weight of trihydrocarbon-substituted and tetrahydrocarbon-substituted ammonium hydroxide. Preferred are tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide. The described anisotropic etching method for semiconductor substrates does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 4,137,123, the method of etching a textured surface into silicon is described using anisotropic etchant. The etchant provides a textured surface of randomly spaced and sized pyramids on a silicon surface and is composed of 0.5-10% by weight silicon and aqueous solutions of alkali metal hydroxides or ammonium hydroxide which optionally contains monohydric, dihydric or polyhydric alcohol where preferably solutions of potassium hydroxide containing isopropyl alcohol or ethylene glycol are employed. The described anisotropic etching method for silicon does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 4,113,551, the method of etching polycrystalline silicon with aqueous solution of NR4OH, where R is an alkyl group is described. Alternate etching solutions for the polycrystalline silicon may consist of aqueous solutions of N(Rm)(H)4-mOH where R is an alkyl group and m is an integer from zero to four, having a molar concentration in the range from 0.0001 moles to the solubility limit or also, aqueous solution of a monoamine selected from the group consisting of R—NH2, R2NH, R3N, RaRbNH and (Ra)2RbN, where R, Ra and Rb are alkyl functional groups and Ra≠Rb. The described anisotropic etching method for polycrystalline silicon does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
As illustrated in U.S. Pat. No. 3,738,881, the method of anisotropically etching silicon and germanium is described using a novel etchant comprised of a strongly alkaline aqueous solution, an oxidizing agent, and a passivating alcohol. The etchant will etch germanium at a high rate with the same degree of geometry control as for silicon. The alkaline etchants proposed in the invention include alkali metal hydroxides such as sodium, potassium, rubidium and cesium hydroxide, as well as quarternary ammonium hydroxides. The described anisotropic etching method for silicon and germanium does not propose using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.
Note that the above methods for anisotropically etching single crystal silicon or etching polycrystalline silicon do not envision, nor describe a method of using high purity aqueous NH4OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH3 gas from aqueous NH4OH solution, for achieving a high anisotropic etching rate.